Download Lecture 11: Matter, atoms

Conceptual Physics
Matter
Atoms
Lana Sheridan
De Anza College
July 14, 2016
Last time
• projectile motion
• orbits
• escape speed
• heliocentric model
• Kepler’s laws
• review
Overview
• matter
• atoms
• elements
• atomic structure
• charge
Dividing Matter
What happens when you cut a loaf of bread in half and then in
half again, and again, and again...?
Dividing Matter
What happens when you cut a loaf of bread in half and then in
half again, and again, and again...?
Can you just keep cutting bread forever?
Dividing Matter
What happens when you cut a loaf of bread in half and then in
half again, and again, and again...?
Can you just keep cutting bread forever?
Or is there some smallest bit of bread, that you can’t cut further?
Dividing Matter
What happens when you cut a loaf of bread in half and then in
half again, and again, and again...?
Can you just keep cutting bread forever?
Or is there some smallest bit of bread, that you can’t cut further?
Or at some point, does it stop being bread?
Some History
People have speculated on the underlying nature of matter for far
longer than there has been good evidence to decide the question.
Ancient Greece:
• Plato (c. 427-c. 347 BCE) thought there were 4 basic kinds
of corpuscles, each in a different geometric shape
• Aristotle (sometime before 330 BCE) decided that the 4 basic
elements (earth, water, fire, air) were continuous
• Democritus, Leucippus, and Epicurus (∼ 500-270 BCE)
believed all matter was composed of small indivisible particles
and that there was empty space, “void”, in between them to
allow for motion
Some History
People have speculated on the underlying nature of matter for far
longer than there has been good evidence to decide the question.
Ancient Rome:
• Lucretius in De rerum natura observes that everything decays,
but that everything can also be reconstituted, suggesting that
the underlying building blocks of matter are eternal, heavily
influenced by Epicurus and Democritus
Some History
Ancient India:
• thinking about atomic concepts seems to have sprouted from
reasoning about linguistics in India
• anor (atom) is mentioned in the Bhavagad Gita (∼500-200
BCE)
• Nyaya-Vaisesika school developed an early idea about atoms
(∼600-0 BCE) and how they combine
• Buddhist school (prior to ∼400 BCE?) had a concept of 4
kinds of atoms very similar to Aristotle’s idea of 4 elements
• Jaina school (600-500 BCE) conceived of atoms as the basic
building blocks of matter, and had some concepts of
sub-atomic constituents
Some History
People have speculated on the underlying nature of matter for far
longer than there has been good evidence to decide the question.
Ancient China:
• atomism seems not to have been a popular idea in ancient
China, but the Mohist school of thought (470 BC-c.391 BCE)
in the Mo Jing speculates that there is an indivisible smallest
unit in the context of Geometry.
Some History
Later thinkers in the Middle East were influenced by Greek and
Indian ideas.
In Europe philosophers were exposed to Greek and Roman ideas,
with Aristotle very popular for a long time.
Nevertheless, ideas about “corpuscles” (small particles) were
important for Newton and his contemporaries.
A chemist, Robert Boyle (1627-1692) speculated that if atoms /
corpuscles made up matter, that much resolve problems arising in
chemistry.
He proved correct.
Understanding from Chemical Reactions
John Dalton, a physicist and chemist in ∼1803 started trying to
understand the patterns of chemical reactions.
Dissociating pure water:
water −→ oxygen + hydrogen
and always hydrogen and oxygen were produced in the same
proportions!
This lead him to suppose that
• matter was composed of atoms
• chemical substances that could not be dissociated were
elements
• chemical substances that could be dissociated were formed
from combinations of atoms
Understanding from Chemical Reactions
Rules of Dalton’s theory1 :
• Elements are made of extremely small particles called atoms.
• Atoms of a given element are identical in size, mass, and
other properties; atoms of different elements differ in size,
mass, and other properties.
• Atoms cannot be subdivided, created, or destroyed.
• Atoms of different elements combine in simple whole-number
ratios to form chemical compounds.
• In chemical reactions, atoms are combined, separated, or
rearranged.
1
Wikipedia, Dalton, “A New System of Chemical Philosophy” (1808)
Understanding from Chemical Reactions
1
Images from Dalton, “A New System of Chemical Philosophy” (1808) and
Wikimedia.
Brownian Motion
In 1827 Robert Brown, a botanist, observed pollen grains
suspended in water through a microscope.
He expected to see them suspended at rest, but did not.
Instead the grains of pollen seemed to jump and wiggle about for
no discernible reason.
Brownian Motion
In 1827 Robert Brown, a botanist, observed pollen grains
suspended in water through a microscope.
He expected to see them suspended at rest, but did not.
Instead the grains of pollen seemed to jump and wiggle about for
no discernible reason.
He wondered if it was something peculiar that pollen did so he
tried again with dust and soot – and saw the same thing!
This motion is called Brownian Motion.
Brownian Motion
Brownian motion remained a mystery until 1905.
Einstein, building on tools he had just developed in his doctoral
thesis, developed a theory describing Brownian motion.
It is the result of fast-moving water molecules (too small to see)
colliding with the pollen molecules, and jostling them.
About Atoms
Facts about atoms:
• atoms are very small: ∼ 10−10 m
• there are 1023 in just a gram of water!
• they move around when not at absolute zero temperature
(which is always)
• atoms of heavier elements are formed in stars. Most atoms
are very stable and therefore almost as old as the universe.
Elements
Atomic Structure
Atoms are made up of subatomic particles:
• protons
• neutrons
• electrons
Atoms of different elements have different
numbers of protons.
1
Figure from Wikipedia.
Electric Charge
Charge is an intrinsic (essential) property of subatomic particles.
Charge can be positive or negative.
Particles can also be “chargeless”, ie. have zero net charge.
Electric Charge
Charge is an intrinsic (essential) property of subatomic particles.
Charge can be positive or negative.
Particles can also be “chargeless”, ie. have zero net charge.
One way we notice charge: in dry weather, it is easy to get a shock
from static electricity.
This is due to a charge imbalance.
Charge on larger objects
562
F
+++
+ + ++++ + +
Glass
++
Most large objects around us have (approximately)
zero net charge.
++
+ + ++++ +
Glass
++++++ + +
CHAPTER
21 ELECTRIC
CHARGE
–F
Objects
can become
charged when
rubbed against one another.
(a)
These examples reveal that w
carpets, doorknobs, faucets, and co
amount of electric charge. Electr
fundamental particles making up t
automatically with those particles w
F
The vast amount of charge in
+
+
the
object
equal amounts
+
++
+ contains
Glass
Glass
++++++++ + +
++++++++ + +
negativeFcharge.
–F With such an equa
+ Glass
– –– Plastic
to be
neutral; that is, i
+ + ++++
–––––––– –
––––electrically
++++++++ + +
charge
are
not
in
balance, then th
–F
charged to indicate
that it has a cha
(b)
(a)
always
much
smaller
than
Fig. 21-2 (a) Two charged rods
of thethe to
1
charge
contained
in
the
object.
Diagrams from Halliday, Resnick, Walker,
9threpel
ed. each other. (b) Two
same sign
Electrostatic force
Charged objects exert a force (the electrostatic force) on one
another.
Charges with the same electrical sign repel each other.
Charges with opposite electrical signs attract each other.
Electrostatic force
Charged objects exert a force (the electrostatic force) on one
another.
Charges with the same electrical sign repel each other.
Charges with opposite electrical signs attract each other.
The unit for charge is the Coulomb, written with the symbol C.
Fundamental Unit of Charge
The smallest unit of charge is called e.
e = 1.602 × 10−19 C
Electrons
Symbol: e−
Negatively charged: charge −e
Very low mass: 9.11 × 10−31 kg
Protons
Symbol: p (or p+ )
Positively charged: charge +e
Much larger mass than an electron: 1.673 × 10−27 kg
Protons
Symbol: p (or p+ )
Positively charged: charge +e
Much larger mass than an electron: 1.673 × 10−27 kg
This is 1837 times the mass of the electron!
Neutrons
Symbol: n (or n0 )
The neutron has no electric charge. (It is neutral.)
Very similar mass to a proton: 1.675 × 10−27 kg. (Just slightly
heavier.)
Atomic Structure
Atoms have a compact nucleus composed of protons and
neutrons.
The nucleus therefore has an overall positive charge.
A cloud of electrons surrounds the nucleus, attracted by the
electrostatic force.
1
Figure from Wikipedia.
Nuclei
Usually, atoms have a similar number of neutrons to the number of
protons in their nuclei.
1
Figure from UC Davis ChemWiki.
Kinds of Atoms
Atoms are neutral (net charge 0), so each atom has the same
number of protons and electrons.
The atomic mass is the sum of the number of protons and
neutrons (×1.67 × 10−27 kg). Electrons can be neglected because
they have so little mass.
Kinds of Atoms
Element
Hydrogen
# Protons/Electons
1
# Neutrons
0
Atomic Mass
1
Helium
2
2
4
Lithium
3
4
7
Berylium
4
5
9
Boron
5
6
11
Carbon
6
6
12
Nitrogen
7
7
14
Oxygen
8
8
16
Fluorine
9
10
19
Neon
10
10
20
Elements
How to read each element square
1
Diagram from WikiPhoto, WikiHow
How to read each element square
Boron has a mass of 11 amu or “atomic mass units”.
1 amu = 1/12 of the mass of one carbon atom.
1
Diagram from WikiPhoto, WikiHow
Summary
• matter
• atoms
• atomic structure
• elements
Homework
• charge worksheet (see website or course studio)
Hewitt,
• Ch 11, onward from page 211. Ranking: 1; Exercises: 1, 9,
11, 13, 15, 17, 21, 27